text: In applying the photoacoustic effect there exist various modes of measurement. Gaseous samples or condensed phase samples, where the pressure is measured in the surrounding gaseous phase, are usually probed with a microphone. The useful applicable time-scale in this case is in the millisecond to sub-second scale. Most often, In this case, the exciting light is continuously chopped or modulated at a certain frequency (mostly in the range between ca. 10–10000 Hz) and the modulated photoacoustic signal is analyzed with a lock-in amplifier for its amplitude and phase, or for the inphase and quadrature components.  When the pressure is measured within the condensed phase of the probed specimen, one utilizes piezoelectric sensors inserted into or coupled to the specimen itself. In this case the time scale is between less than nanoseconds to many microseconds 
  The photoacoustic signal, obtained from the various pressure sensors,  depends on the physical properties of the system, the mechanism that creates the photoacoustic signal, the light-absorbing material, the dynamics of the excited state relaxation and the modulation frequency or the pulse profile of the radiation, as well as the sensor properties. This calls for appropriate procedures to (i) separate between the signals due to different mechanisms and (ii) to obtain the time dependence of the heat evolution (in the case of the photothermal mechanism) or the oxygen evolution (in the case of the photobaric mechanism in photosynthesis) or the time dependence of the volume changes, from the time dependence of the resulting photoacoustic signal.